Method of fabricating epitaxial semiconductor material

Abstract: An epitaxial semiconductor material particularly suitable for a high switching rectifier has misfit dislocations (28, 30, 32) formed in the depletion region (16) adjacent a PN junction (20). A method is described for forming each Si/Ge misfit dislocation region (Fig. 2) epitaxially on a capped Si substrate (10). These misfit dislocation layers are formed in a step in a CVD reactor chamber at a lowered temperature. To enhance switching speed gold, and preferably gold and platinum, is diffused through the Si body and forms relatively large concentrations at the misfit dislocation sites. In a modification (Fig. 3) the misfit dislocations are provided just outside the depletion region (16).

Description: The present invention relates to a method for fabricating a structure of semiconductor material by epitaxial growth on a silicon substrate in a CVD reactor chamber. The invention also relates to a method of forming a bipolar semiconductor device including a power rectifier junction. The invention is concerned with the fabrication of epitaxial material in which misfit dislocations are formed. The use of misfit dislocations permits improved control of the switching speed or other lifetime dependent parameters of bipolar semiconductor devices manufactured from such semiconductor material. It is known that the switching speed of silicon bipolar devices can be increased by the introduction of heavy metallic impurities in the active region of the device. The impurities significantly reduce minority-carrier lifetime and as a result shortens reverse recovery time. Gold and platinum impurities are often used, and are rapidly diffused into this silicon. However, the solid solubilities of these impurities in silicon are very small and they tend to precipitate or segregate at defects, disordered regions, and surfaces. Their diffusion is therefore difficult to control and is sensitive to process conditions, particularly the diffusion temperature and time, and also cooling rate (after the diffusion) in certain temperature ranges. U.S. Patent No. 5,097,308 issued March 7, 1992 and U.S. Patent No. 5,102,810 issued April 7, 1992, both to Ali Salih, are directed to a method for controlling the switching speed of bipolar power devices through the use of misfit dislocations in the depletion region. The misfit dislocations are formed during epitaxial growth by creating a silicon/silicon-germanium interface. The above-mentioned U.S. patent 5,102,810 discloses a method for fabricating a structure of semiconductor material by epitaxial growth on a silicon substrate in a chemical vapor deposition (CVD) reactor chamber, in which a region having misfit dislocations is formed by successively growing layers of germanium-containing silicon and germanium-free silicon on an initial epitaxially grown silicon layer. It is to be recognized that this initial layer will be grown at an appropriate given temperature. In US patent 5,102,810 there is disclosed a semiconductor structure having a region of misfit dislocations, within which there are two or more spaced germanium-containing silicon layers, these layers being separated by a germanium-free silicon layer. Both the mentioned U.S. patents propose the use of misfit dislocations to lower the minority lifetime in semiconductor devices. This can be used to enhance the switching speed of power rectifiers, particularly fast recovery power rectifiers. With misfit dislocations it becomes possible to obtain localized regions in the device with low minority lifetime close to regions with a high minority lifetime. It also allows for a great amount of freedom to place these misfit dislocations in favourable locations. Misfit dislocations produce relatively small leakage currents and are metallurgically and electrically stable, and thus, device performance does not significantly deteriorate under high power and temperature applications. Unlike processes where misfit dislocations had been previously used to remove impurities from the space charge region, the use of misfit dislocations in the two U.S. patents is to directly reduce minority carrier lifetime. That becomes possible because they are associated with deep energy levels in the energy gap of silicon, like metallic impurities. The misfit dislocations are formed by growing a thin layer of silicon containing a few percent germanium (Ge) somewhere in the epitaxial layer of "pure" silicon (substantially Ge free). The two interfaces of the silicon without Ge and the thin layer of silicon containing a few percent Ge are strained, because the crystal lattices of the bordering layers do not exactly match. As a result dislocations appear at these interfaces. Unlike metal impurities and structural defects formed by electron beam irradiation, misfit dislocations are permanent defects that are not sensitive to high temperature processing short of melting the silicon material. They are also localized, having their influence in well defined narrow regions in the Si device. They are formed during the epitaxial growth and do not require additional processing steps as ones used to introduce metal impurities and structural defects. Thus, misfit dislocations are used effectively as a more versatile alternative to metal diffusion with the added advantage of localization of their effects. The misfit dislocations can also be used as a complement to metal diffusion to obtain significantly better results than conventional impurity introduction techniques. Flexible tailoring of recovery characteristics can be achieved by adjusting the placing of the dislocations as well as a adjusting the densities of both misfit dislocations and metal impurities combined in the same device. The above described patented process teaches forming the silicon/silicon-germanium interface during epitaxy by growing a silicon layer with 2% or 3% germanium and thereafter growing a "pure" silicon layer. However, as a practical matter, it is difficult to achieve a "pure" (substantially germanium free) silicon layer after the silicon-germanium layer is formed because of residual germanium which remains in the reactor system. The present invention enables us to provide a method whereby silicon layers which are not contaminated with germanium can be formed after silicon-germanium layers are fabricated in a CVD reactor during epitaxy. According to the present invention there is provided a method for fabricating a structure of semiconductor material by epitaxial growth on a silicon substrate in a chemical vapor deposition (CVD) reactor chamber, in which a region having misfit dislocations is formed by successively growing layers of germanium-containing silicon and germanium-free silicon on an initial silicon layer epitaxially grown at a given temperature, and wherein the formation of said region is characterized by the steps of: lowering the temperature of said substrate to about 50°C below said given temperature; growing at least one silicon layer containing about 2-3% germanium with the aid of a germanium-containing gas introduced into the reactor chamber; purging the reactor chamber and raising the temperature of said substrate to said given temperature; growing a first germanium-free silicon layer on the, or the last-formed, germanium-containing silicon layer; purging the reactor chamber; growing a second germanium-free silicon layer; etching said second germanium-free silicon layer to remove a part thereof; and purging the reactor chamber. The prior method teaches placement of the silicon/silicon-germanium interface near the middle of the depletion region. The present invention provides for the possibility of placement of the misfit dislocation regions near or even outside the boundary of the depletion layer. Preferred features of the practice of the invention are concerned with the provision of more than one region of misfit dislocations; the inclusion of more than one germanium-containing silicon layer in a misfit dislocation region; the formation of a P/N junction in the epitaxially-grown structure; and the placement of one or more misfit dislocation regions with respect to the junction or a depletion layer associated therewith. Preferred aspects of the above method of the invention are set forth in Claims 2 to 11. One of these preferred aspects relates to a method of forming a bipolar semiconductor device including a power rectifier P/N junction. This method includes fabricating a structure of semiconductor material having a P/N junction formed therein by a method as defined in claim 11, and incorporating the structure into a bipolar semiconductor device in which the P/N junction provides a power rectifier junction for the device. The prior method suggests the diffusion of metallic impurities as a complement to the use of misfit dislocations. It is now suggested that the simultaneous diffusion of gold and platinum impurities appears to provide optimum results. It is further proposed that the placement of an Au doped wafer in the reactor chamber during the growing of the epitaxial layers in the region of misfit dislocations, amidst the substrate wafers which are being processed, will be a useful source of Au. The grown epitaxial layers pick up enough Au, which diffuses and ends up mainly in the misfit dislocation regions. As the reader will no doubt note, the process here disclosed is entirely epitaxial in nature and can be performed completely in a single CVD reactor. In that sense, it is similar to the process disclosed in Co-pending European Application 94301701.2 published as EP-A-0671770 on 13-09-95. EP-A-0671770 teaches an all epitaxial process in which a heavily doped layer is grown on a lightly doped substrate. To minimize out diffusion, the substrate is "capped" by growing very thin and heavily doped silicon layers which are depleted by hydrogen purges. The epitaxial layers are then grown over the capped substrate. Those aspects are effectively utilized in the preferred practice of the present invention which combines same with some of the features of the process of the U.S. patent already discussed to obtain a more advantageous result. To minimize contamination of the gas flow lines and reactor surfaces from the high impurity concentrations necessary in the CVD reactor when performing the epitaxial process, advantageously a gas flow system is employed with the CVD reactor, which system is that described in copending European Application 94301700.4, published as EP-A-0671484 on 13-09-95. The preferred practice of the present invention described below provides a semiconductor material in which germanium contamination of silicon layers, which are formed in the CVD reactor after germanium-containing layers, is substantially reduced. The minority lifetime is controlled, and the switching speed of devices made using the semiconductor material structure is enhanced - preferably gold and platinum impurities are diffused simultaneously as a complement to the misfit dislocation process. A gold diffusion may be performed during the epitaxial growing steps by replacing one of the wafers in the epitaxial susceptor by a gold doped wafer as a source of gold for the other wafers. The method of the invention may be put into practice so that the misfit dislocations are located in a region proximate to or outside a depletion region or outside a space charge region in the case of a forward biased junction. In accordance with one embodiment of the present invention, a method is provided for fabricating an improved epitaxial material (see Fig. 2). In the CVD reactor, an epitaxial layer is grown on a silicon substrate. The layer includes a first region containing misfit dislocations. Germanium-containing gas is introduced into the reactor chamber as the substrate temperature is reduced to 1150°C to grow a 2µm thick silicon layer containing for example 3% germanium. Next the chamber is then purged. A layer of substantially germanium-free silicon is then grown again, 2µm thick. Germanium-containing gas is again introduced into the chamber to form a second germanium containing layer, 2µm thick. The chamber is purged after the second germanium-containing layer is formed. During this purge the temperature is increased. A second substantially germanium free silicon layer (2µm thick) is grown on the surface of the second germanium containing layer. The chamber is purged again. Next another 2µm substantially germanium-free silicon layer is formed. Finally the surface of this last layer, is etched to remove a significant part of it. HCl gas is introduced to back etch about one micrometer of the silicon layer surface. This is to remove unwanted germanium residue which has settled on the wall of the vapor deposition chamber thereby preventing further unwanted silicon germanium deposition. A second region of several misfit dislocations may be formed at a location spaced from the first region. A third, region of misfit dislocations may be formed at a location spaced from the second region. The material may then be subjected to diffusion to form a P/N junction. The method further comprises the step of simultaneously introducing gold and platinum impurities into the first region. The present invention may be used to provide a material with which to fabricate a bipolar power semiconductor device. The material includes a silicon substrate with first and second surface areas. An epitaxial layer is formed on the substrate. The epitaxial layer includes a layer that in the finished device will become a depletion region of a first conductivity type. A region in the epitaxial layer is formed under conditions which result in misfit dislocations as described above. The present invention and its practice will now be further described with reference to the accompanying drawings, wherein like numerals relate to like parts and in which: Fig. 1 is a schematic cross-sectional view of a portion of a bipolar power semiconductor device formed in a material fabricated according to the method of the present invention, and illustrating the placement of three spaced regions of misfit dislocations; Fig. 2 is a schematic cross-sectional view of one of these three regions of misfit dislocations shown in Fig. 1; and Fig. 3 is a schematic cross-sectional view like Fig. 1, of another device formed of a material fabricated in accord with the present invention. The devices formed using a semiconductor material fabricated according to the method of this invention utilize misfit dislocation regions formed in or close to a depletion region in the device. The dislocation regions function as an alternative and a complement to metal diffusion for enhancement of switching characteristics of power rectifiers. The misfit dislocations are formed in a silicon epitaxial layer in the depletion region in material which will become a bipolar power semiconductor device, such as a rectifier, before the rectifying junction is diffused. The misfit dislocations serve as lifetime killers and also as getter sites for metallic impurities. A small amount of metallic atoms, preferably a mix of gold and platinum, will be simultaneously diffused into the misfit dislocation region and will remain there in larger concentrations than outside the misfit dislocation regions, leaving the surface and the remaining depletion region with minimal metallic precipitates. The end result, if the place for the misfit dislocation region is well chosen, is a very fast switching rectifier with relatively low forward voltage and leakage current. A process embodying the present invention has been successfully demonstrated with a rectifier having a switching speed about 25 nanoseconds and a breakdown voltage of approximately 700 to 800 volts. The process is particularly well suited for use in the production of high voltage, very fast switching power rectifiers rated at less than 1000 volts. Description of Preferred Embodiment By way of an example, the process may start with an N type substrate 10 (see fig. 1) of Arsenic doped silicon of less than .005 ohm-cm resistivity with crystal orientation of 〈1-0-0〉. The substrate is preferably capped as taught in EP-A-0671770 referred to above. A N- silicon epitaxial layer is grown (in a vapor deposition reactor chamber) over the substrate. In this epitaxial layer three regions are grown with misfit dislocations. The placement of the regions is such that when processing is finished, these misfit dislocation regions end up in or close to the depletion region, which is a fully reverse biased P-N junction. Each misfit dislocation region consists of two 2 micrometer thick layers of silicon with 3% germanium, separated by a 2 micrometer layer of silicon with no germanium (see fig.2). Two layers of two micrometers thick silicon sub layers are deposited on top of the second SiGe layer. More particularly, above capped surface 12 of substrate 10 the epitaxial layer is grown. After the device is finished the epitaxial layer will include an N++ out diffusion buffer region 14. A P+ diffused region 18 is diffused into this epitaxial layer to create a P-N junction 20. If this P-N junction is reverse biased, a depletion region 16 will sustain the bias voltage. Depletion region 16 will contain N-type silicon with regions 22, 24 and 26 separated by regions 28, 30 and 32 containing misfit dislocations. Each of the misfit dislocation regions 28, 30 and 32 are preferably formed of five layers each approximately 2 micrometers thick. A typical region 28, 30 and 32 including misfit dislocations is illustrated in Figure 2. With the temperature of the CVD reactor chamber lowered 50°C to 1050°C, a first germanium containing layer 34 (Si/Ge) of approximately 3% germanium is grown on an initial, underlying layer of epitaxial growth as seen in Fig. 1. The germanium-containing layer is approximately 2 micrometers thick. Next a 4 min. hydrogen purge is performed to clean the gas in the chamber. A first substantially germanium free silicon layer 36 is then grown over layer 34. Layer 36 is also approximately 2 micrometers thick. On top of layer 36 a second germanium containing layer 38 is grown (Si/Ge) which is similar to layer 34 in composition and thickness. After this another purge. During this purge the temperature is raised to 1100°C. Above layer 38 another substantially germanium free layer 40 is grown. Then another purge and another substantially germanium free layer 42. Silicon sublayers 40 and 42, each are approximately two micrometers thick. Finally a gas etch etches away a part of layer 42 and another 4 min. hydrogen purge removes the etching gasses. Each multi-layer region of misfit dislocations has a thickness of approximately 10 micrometers. The regions are spaced apart by N-silicon regions 24 and 26 (Fig.1) of approximately 13 micrometers thickness. Adjacent the surface of region 32 of misfit dislocations is grown an approximately 45 micrometer thick silicon buffer. A portion of the buffer will be diffused with a P type impurity to form a P+ type region 18 with a P/N junction 20. The regions of misfit dislocations are preferably proximate to junction 20. Fig. 3 shows the case where the misfit dislocation regions 28 and 30 are situated just outside the depletion region 16. The misfit dislocation regions are the same as earlier described and shown in Fig.2, however preferably with one, two or more Ge doped layers. The advantage of this semiconductor material structure is found primarily in the leakage current IR and to a lesser extent to the forward voltage drop Vf. Hole-electron generation in the depletion layer is by far the largest component of the IR. Diffusing metallic impurities all through the wafer to lower the minority carrier lifetime (as is currently the standard manner of achieving this) is known to increase the IR because the generation component of IR is proportional to the lifetime. IR (gen) = q ni (Vol. of depl region)/n.τ. where IR (gen) is the generation part of the leakage current. ni is the intrinsic carrier concentration. q is the electron charge (1.6 x 10 -19 C) τ is the lifetime n is a constant. For a midband energy level n = 2 If the τ in the depletion region is large, IR is small. If τ just outside the depletion region is small, the recombination there will diminish the charge in the space charge region when the rectifier is forward biased, which will result in a faster switching time. The misfit dislocations in this case must have more regions or more germanium in each 2µm region or more gold or/and Pt to achieve the same switching time as in the case of fig. 1. In some cases only one of the misfit dislocation regions might be the preferred embodiment. A similar reasoning is in special cases true for the space charge region. As noted above, increasing the switching speed of silicon devices has been conventionally accomplished by the introduction of heavy metallic impurities, which tend to reduce minority carrier lifetime and therefore result in devices with a shorter recovery time but with a higher IR due to the metallic impurities such as gold and platinum which have been used. In a special case of the implementation of the present invention, misfit dislocations as well as metal diffusion are employed to enhance the switching speed in devices such as power rectifiers. The misfit dislocations serve as getter sites of the metallic impurities. The metallic impurities are diffused all through the silicon and end up into the misfit dislocation regions as the regions are formed and remain in those regions in relatively large concentrations, as compared to the surface and remainder of the depletion region. The source of these metallic impurities can be a gold doped wafer in the vapor deposition reactor chamber. We have found that maximum results are obtained by simultaneously diffusing gold and platinum impurities into the misfit dislocation regions after the regions are formed. This can be achieved with a gold and platinum Filmtronic spin-on used as a metallic dopant. It is believed that, because the metals have different activation energies, different and distant trap energies are present. Diffusion of the gold and platinum at 810° C for 60 minutes has resulted in power rectifiers with a switching time in the 25-30 nano second range. It should now be appreciated that described above is a practical application of misfit dislocation techniques. The misfit dislocations are associated with deep energy levels in the energy gap of silicon. Misfit dislocations act as localized lifetime killers. The employment of such misfit dislocations is a controllable, clean technique for increasing switching speed of power rectifiers and reducing the minority carrier lifetime of multilayer devices. In addition, misfit dislocations are effective gettering sites for metallic impurities. Small amounts of gold and platinum can easily be incorporated into the region of misfit dislocations, leaving the remaining silicon layers with minimal metallic impurity precipitates.

Claims: A method for fabricating a structure of semiconductor material by epitaxial growth on a silicon substrate (10) in a chemical vapor deposition (CVD) reactor chamber, in which a region (28:30:32) having misfit dislocations is formed by successively growing layers (34, 36, 38) of germanium-containing silicon and germanium-free silicon on an initial silicon layer (14:24:26) which is epitaxially grown at a given temperature, and wherein the formation of said region (28:30:32) is characterized by the steps of: lowering the temperature of said substrate (10) to about 50°C below said given temperature; growing at least one silicon layer (34:38) containing about 2-3% germanium with the aid of a germanium-containing gas introduced into the reactor chamber; purging the reactor chamber and raising the temperature of said substrate (10) to said given temperature; growing a first germanium-free silicon layer (40) on the, or the last-formed, germanium-containing silicon layer (34:38); purging the reactor chamber; growing a second germanium-free silicon layer (42); etching said second germanium-free silicon layer (42) to remove a part thereof; and purging the reactor chamber. A method as claimed in Claim 1 in which more than one germanium-containing silicon layer (34, 38) is grown and in which between successive growths of germanium-containing silicon layers (34, 38), the reactor chamber is purged and a germanium-free layer (36) of silicon is grown. A method as claimed in Claim 1 in which two or more spaced regions (28, 30, 32) of misfit dislocations are formed, each being formed by successively growing layers (34, 36, 38) of germanium-containing silicon and germanium-free silicon on a respective initial silicon layer (14, 24, 26) which is epitaxially grown at said given temperature, and wherein the formation of each of said regions is characterised by the steps of Claim 1. A method as claimed in Claim 3 in which there are three regions (28, 30, 32) of misfit dislocations are formed. A method as claimed in Claim 3 or 4 in which in the formation of each of said regions, more than one germanium-containing silicon layer (34, 38) is grown, and in which between successive growths of germanium-containing silicon layers (34, 38) in each region, the reactor chamber is purged and a germanium-free silicon layer (36) is grown. A method as claimed in any preceding claim further comprising the step of growing an epitaxial silicon layer above the misfit dislocation region(s) and forming a P/N junction (20) in the last-mentioned epitaxial silicon layer. A method as claimed in Claim 1 or Claim 2 comprising the step of forming a P/N junction (20) in the epitaxially grown structure, said P/N junction (20) having a depletion region (16) associated therewith, and in which said region (28:30) of misfit dislocation is located just outside said depletion region (20) proximate one of the boundaries of said depletion region. A method as claimed in Claim 3 or Claims 3 and 5 comprising the step of forming a P/N junction (20) in the epitaxially-grown structure, said P/N junction (20) having a depletion region (16) associated therewith, and in which respective regions (28, 30) of misfit dislocations are formed just outside the depletion region (16) proximate respective ones of the boundaries thereof. A method as claimed in any preceding claim further comprising the step of simultaneously diffusing gold and platinum impurities into the or each region (28:30:32) of misfit dislocations. A method as claimed in any one of Claims 1 to 8 further comprising the step of diffusing gold as an impurity into the or each misfit dislocation region (28:30:32) and obtaining the gold impurity for diffusion from a gold-doped wafer in the reactor chamber. A method in accordance with Claim 6, 7 or 8, further comprising: incorporating said semiconductor structure including said P/N junction into a bipolar semiconductor device in which said P/N junction (20) provides a power rectifier junction for the device.

Inventors: CHAN JOSEPH Y, EINTHOVEN WILLEM G, GARBIS DENNIS, LATERZA LARRY, CHAN, JOSEPH Y., EINTHOVEN, WILLEM G., GARBIS, DENNIS, LATERZA, LARRY
Applicants: GEN SEMICONDUCTOR INC, GENERAL SEMICONDUCTOR, INC.
Application year: 1994
Main classification: H01L21
IPC codes: H01L21, H01L29